Question

Give a brief description of the principles of the Crystallisation and give examples.

Answer 1

Crystallisation is used to purify organic compounds.
It utilizes the difference in solubility of the compound and the impurities. The compound to be crystallized is sparingly soluble in solvent at room temperature but significantly soluble at high temperature. A saturated solution is obtained by concentrating the solution. On cooling, pure compound crystallizes out and the impurities remain in the solution. Pure compound is obtained by filtration.
Thus, crude aspirin is recrystallized to obtain pure aspirin. About 3 g of crude aspirin is dissolved in 20 mL of ethanol. The solution is heated which ensures complete dissolution. It is then slowly allowed to cool. The crystals are then filtered.